Silicon nitride substrate, method of manufacturing the same, and silicon nitride circuit board and semiconductor module using the same

ABSTRACT

A silicon nitride substrate having appropriately adjusted warpage and surface roughness can be obtained by mixing magnesium oxide of 3 to 4 wt % and at least one kind of rare-earth element oxide of 2 to 5 wt % with silicon nitride source material powder to form a sheet-molded body, sintering the sheet-molded body, and performing a heat treatment at a temperature of 1,550 to 1,700 degree C. with a pressure of 0.5 to 6.0 kPa with a plurality of substrates being stacked. Also, a silicon nitride circuit board and a semiconductor module using the same are provided.

BACKGROUND

1. Technical Field

The present invention relates to a silicon nitride substrate and amethod of manufacturing the same. In addition, the present inventionrelates to a silicon nitride circuit board and a semiconductor moduleusing the silicon nitride substrate.

2. Description of the Related Art

Recently, in the field of an inverter for an electric vehicle or thelike, power semiconductor modules (such as IGBT or a power MOSFET)capable of operating with a high voltage and a large electric currentare employed. The power semiconductor module may include an insulativeceramic circuit board having a metal circuit plate on its one surfaceand a metal heat sink plate on the other surface. In addition, asemiconductor device is mounted on the superior surface of the metalcircuit plate. The insulative ceramic substrate is bonded to the metalcircuit plate and the metal heat sink plate by means of, for example, anactive metal method using brazing filler metal or a direct copperbonding method in which a copper plate is directly bonded.

Since such power semiconductor module generates a large amount of heatby flowing a large amount of current, a thermal stress is generated dueto a difference of thermal expansion rates between the insulativeceramic substrate and the metal circuit plate or between the insulativeceramic substrate and the metal heat sink plate. This may cause fracturethat generates cracks on the insulative ceramic substrate or exfoliationof the metal circuit plate or the metal heat sink plate from theinsulative ceramic substrate. The insulative ceramic substrate may bemade of, for example, aluminum nitride or silicon nitride. However,since the insulative ceramic substrate made of aluminum nitride has alow mechanical strength, it may be susceptible to the cracks orexfoliation, so that may not be suitable to be used in the powersemiconductor module.

In this regard, Japanese Patent Application Laid-Open (JP-A) No.11-268958 discloses an example of a sintered silicon nitride substrate.According to the patent document, an inner layer of the substrate has afine grain structure, and an outer layer has a combinational structurecontaining both coarse and fine grains, thereby improving strength andtoughness. Japanese Patent Application Laid-Open (JP-A) No. 61-186257discloses a silicon nitride ceramic structure, in which sizes of ceramicgrains contained in a surface layer are larger than the sizes of theceramic grains contained in an inner layer, thereby improving strength.In Japanese Patent Application Laid-Open (JP-A) No. 61-10069, finepowder of magnesium carbonate MgCO₃ or magnesium hydroxide Mg(OH)₂,which is thermally decomposed into magnesium oxide MgO, is used as asintering additive for forming grain boundary phases, thereby obtaininga sintered body having grain boundary phases regularly diffused. Thus,strength is improved and strength difference is reduced. In JapanesePatent Application Laid-Open (JP-A) No. 2004-161605, a number ofsintering additive components are previously mixed and regularlydiffused, and then, silicon nitride powder as a main source material ismixed with them, so that a sintered body having high strength withsuppressed agglomeration or segregation can be obtained.

However, the techniques disclosed in the aforementioned documents failedto appropriately adjust warpage and surface roughness of the siliconnitride substrate. Generally, when the warpage of the silicon nitridesubstrate becomes large, an adhesion property of the metal circuit plateand the metal heat sink plate is degraded, so that the metal circuitplate and the metal heat sink plate may become susceptible toexfoliation from the silicon nitride substrate due to the thermal stressgenerated during a cooling process from a bonding temperature (at about800 degree C.) between the silicon nitride substrate and the metalcircuit plate, and between the silicon nitride substrate and the metalheat sink plate, or a heating and cooling cycle when operating the powersemiconductor module. Also, when surface roughness of the siliconnitride substrate is large, the surface adhesion property of the siliconnitride substrate with the metal circuit plate and the metal heat sinkplate is degraded, so that the metal circuit plate and the metal heatsink plate may become susceptible to exfoliation from the siliconnitride substrate as described above. Accordingly, it is necessary toappropriately adjust the warpage and surface roughness, but theaforementioned documents do not disclose any technique for adjusting thewarpage and the surface roughness of the silicon nitride substrate.Therefore, as described above, the warpage and the surface roughness ofthe silicon nitride substrate cannot be appropriately adjusted.

SUMMARY

An object of the present invention is to provide a silicon nitridesubstrate having warpage and surface roughness appropriately adjusted, amethod of manufacturing the same, and a silicon nitride circuit boardand semiconductor module using the same.

According to one aspect of the invention, there is provided a siliconnitride substrate containing silicon nitride, wherein a degree oforientation representing an orientation ratio on a plane perpendicularto a thickness direction, determined by a ratio of X-ray diffractionbeam intensity on a predetermined lattice surface of a grain of thesilicon nitride, is 0.33 or less on a surface; and the degree oforientation is 0.16 to 0.33 on a surface obtained by grinding as deep as20% or more of a thickness of the substrate; and warpage is 2.0 μm/mm orless. Since a surface bonded to a metal plate is necessary to satisfysuch requirements on the degree of orientation, it may satisfy therequirements for both surfaces of the silicon nitride substrate.

According to another aspect of the invention, there is provided asilicon nitride substrate containing β-type silicon nitride, yttrium Y,and magnesium Mg, wherein a variation coefficient representingdistribution of magnesium on a surface of the silicon nitride substrateis 0.20 or less, and warpage is 2.0 μm/mm or less. Since a surfacebonded to a metal plate is necessary to satisfy such requirements on thevariation coefficient, it may satisfy the requirements for both surfacesof the silicon nitride substrate.

According to another aspect of the invention, there is provided a methodof manufacturing a silicon nitride substrate, the method including:mixing magnesium oxide of 3 to 4 wt % and at least one kind ofrare-earth element oxide of 2 to 5 wt % with silicon nitride sourcematerial powder to form a sheet-molded body; sintering the sheet-moldedbody; and performing a heat treatment at a temperature of 1,550 to 1,700degree C. with a pressure of 0.5 to 6.0 kPa with a plurality ofsubstrates being stacked.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are illustrative diagrams showing a relationship betweena degree of orientation and a longitudinal length of a columnar grain;

FIG. 2 is a flowchart illustrating an example of a method ofmanufacturing a silicon nitride substrate;

FIG. 3 is an illustrative diagram showing a method of applying a loadduring a thermal process of FIG. 2;

FIG. 4 is another illustrative diagram showing a method of applying aload during a thermal process of FIG. 2;

FIG. 5 is an illustrative diagram showing a blast process of FIG. 2;

FIGS. 6A and 6B are electron microscope pictures showing a surface of asilicon nitride substrate before and after the blast process of FIG. 2;and

FIGS. 7A and 7B are illustrative diagrams showing a method of measuringwarpage.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

An exemplary embodiment of the present invention (referred to as “anembodiment” below) will be described hereinafter.

First Embodiment

According to a first embodiment of the present invention, there isprovided a silicon nitride substrate which is an insulative ceramicsubstrate used in a semiconductor module or the like, wherein thesilicon nitride substrate contains silicon nitride Si₃N₄, a degree oforientation representing an orientation ratio on a surface perpendicularto a thickness direction, determined by a ratio of each X-raydiffraction beam intensity on a predetermined lattice surface of thesilicon nitride grains, is set to 0.33 or less, and the degree oforientation is set to 0.16 to 0.33 on the surface obtained by grindingas deep as 20% or more of the substrate thickness. The warpage of thesilicon nitride substrate is 2.0 μm/mm or less.

Here, the substrate surface refers to the outermost surface of thesilicon nitride substrate obtained after a heat treatment process (d),which will be described later, i.e., the surface before grinding whenthe silicon nitride substrate is manufactured, or the surface obtainedby grinding as deep as 10% or less of the substrate thickness (15 (μm atmaximum) from the outermost surface.

The degree of orientation fa can be represented as follows:

fa=(P−P0)/(1−P0)  (1)

In above Equation (1), P denotes a ratio of the X-ray diffraction beamintensities on each of the plane (110), the plane (200), the plane(210), the plane (310), and the plane (320) of the silicon nitride grainin the silicon.nitride substrate as will be described in Equation (2)later. P0 denotes a ratio of the X-ray diffraction beam intensities oneach of the plane (110), the plane (200), the plane (210), the plane(310) and the plane (320) of the silicon nitride grain in the siliconnitride substrate as will be described in Equation (3).

P=(I ₍₁₁₀₎ +I ₍₂₀₀₎ +I ₍₂₁₀₎ +I ₍₃₁₀₎ +I ₍₃₂₀₎)/(I ₍₁₁₀₎ +I ₍₂₀₀₎ +I₍₁₀₁₎ +I ₍₂₁₀₎ +I ₍₂₀₁₎ +I ₍₃₁₀₎ +I ₍₃₂₀₎ +I ₍₀₀₂₎)  (2)

P=(I′ ₍₁₁₀₎ +I′ ₍₂₀₀₎ +I′ ₍₂₁₀₎ +I′ ₍₃₁₀₎ +I′ ₍₃₂₀₎)/(I′ ₍₁₁₀₎ +I′ ₍₂₀₀₎+I′ ₍₁₀₁₎ +I′ ₍₂₁₀₎ +I′ ₍₂₀₁₎ +I′ ₍₃₁₀₎ +I′ ₍₃₂₀₎ +I ₍₀₀₂₎)  (3)

While the silicon nitride substrate contains both coarse and finecolumnar grains of silicon nitride as a main component, the degree oforientation fa of the substrate surface is determined according to theorientation of the coarse columnar grains. The degree of orientation famay range between −1 and 1. When the degree of orientation fa is 0, itis recognized that the coarse columnar grains are distributed in adisorderly manner. When the degree of orientation fa is larger than 0 asin the silicon nitride substrate according to the present embodiment, itis recognized that the silicon nitride substrate contains a largercontent of columnar grains having an inclination of a long axis withrespect to the thickness direction of the silicon nitride substratelarger than 45 degree. As the degree of orientation fa reaches 1, it isrecognized that the inclination of a long axis with respect to thethickness direction of the silicon nitride substrate reaches 90 degree.

It is recognized that, when the degree of orientation fa is large, thelongitudinal lengths of the columnar grains grow (i.e., become longer).Referring to FIGS. 1A and 1B, it is shown a relationship between thedegree of orientation fa and the longitudinal length of the columnargrain. FIG. 1A shows a case where the degree of orientation fa is large,and FIG. 1B shows a case where the degree of orientation fa is small.Referring to FIG. 1A which shows a case where the degree of orientationfa is large, the longitudinal length of the columnar grains 12 containedin the silicon nitride substrate 10 is larger than that of FIG. 1B whichshows a case where the degree of orientation fa is small. Accordingly,when the degree of orientation fa of the surface of the silicon nitridesubstrate 10 is large, the ratio of the long-length columnar grains 12increases, so that the surface roughness of the silicon nitridesubstrate also increases. If the surface roughness increases, theadhesion property with the metal circuit plate and the metal heat sinkplate is degraded when a power semiconductor module or the like ismanufactured. Therefore, the metal circuit plate and the metal heat sinkplate may become susceptible to exfoliation from the silicon nitridesubstrate 10 during a bonding process to the silicon nitride substrate10 or a heat cycle accompanied by operation of the power semiconductormodule. On the other hand, if the degree of orientation fa inside thesilicon nitride substrate 10 decreases, the ratio of the long-lengthcolumnar grains 12 decreases, and bending strength, fracture toughness,or the like also decreases. As a result, the silicon nitride substrate10 becomes susceptible to cracks during a bonding process to the metalcircuit plate and the metal heat sink plate, or a heat cycle accompaniedby operation of the power semiconductor module. In this case, the degreeof orientation fa inside the silicon nitride substrate 10 is a degree oforientation fa of the surface obtained by grinding as deep as 20% ormore of the substrate thickness as described above. Alternatively, thesurface for measuring the degree of orientation fa inside the siliconnitride substrate 10 may be obtained by grinding the substrate thicknessas deep as 30 μm or more from the substrate surface. In addition, if thedegree of orientation fa inside the silicon nitride substrate 10increases, then the degree of orientation fa of the surface of thesilicon nitride substrate 10 also increases resulting in a problem thatthe surface roughness may also increase. From the reasons describedabove, it is necessary to appropriately adjust the degrees oforientation fa inside as well as on the surface of the silicon nitridesubstrate 10.

Therefore, in the silicon nitride substrate according to the presentembodiment, the degree of orientation on the substrate surface is set to0.33 or less, and the degree of orientation on the surface obtained bygrinding as deep as 20% of the substrate thickness or more from thesubstrate surface is adjusted to 0.16 to 0.33 as described above. As aresult, it is possible to reduce surface roughness of the siliconnitride substrate as well as to improve bending strength and fracturetoughness. How to adjust the degree of orientation fa will be describedlater.

If the warpage of the silicon nitride substrate increases, the adhesionproperty between the silicon nitride substrate and the metal circuitplate or between the silicon nitride substrate and the metal heat sinkplate may be easily degraded in some parts. As a result, the metalcircuit plate and the metal heat sink plate may become susceptible toexfoliation from the silicon nitride substrate. Therefore, in thesilicon nitride substrate according to the present invention, thewarpage is limited to 2.0 μm/mm or less as described above. How to limitthe warpage will be described later.

The silicon nitride substrate according to the present embodimentcontains magnesium Mg of 3 to 4 wt % as magnesium oxide MgO and at leastone kind of rare-earth element oxide of 2 to 5 wt %. Here, therare-earth element oxide may be, for example, yttrium oxide. Since themagnesium and the rare-earth element (e.g., yttrium) function as asintering additive for growing columnar grains of silicon nitride, thegrowth of the columnar grains may be insufficient, and the content ofcolumnar grains having short longitudinal length increases when itscontent is small. Accordingly, the bending strength and the fracturetoughness of the silicon nitride substrate decrease. Meanwhile, when thecontents of magnesium and a rare-earth element increase, the growth ofcolumnar grains is stimulated, so that the content of columnar grainshaving a large longitudinal length increases. Accordingly, the degree oforientation fa of the silicon nitride substrate also increases, and thesurface roughness increases. In this embodiment, each of the contents ofmagnesium and a rare-earth element is set to the aforementioned range toadjust such properties.

Second Embodiment

According to a second embodiment of the present invention, there isprovided a silicon nitride substrate, which is an insulative ceramicsubstrate used in the power semiconductor module or the like asdescribed above, containing β-type silicon nitride, yttrium (Y), andmagnesium (Mg), wherein a variation coefficient which representsdistribution of the amount of magnesium Mg is set to 0.20 or less. Thewarpage of the silicon nitride substrate is set to 2.0 μm/mm or less.

The variation coefficient representing distribution of the amount ofmagnesium is measured by scanning an arbitrary location of the substratesurface within a range of 1 mm by irradiating an EPMA beam having adiameter of 1 μm, measuring values of an X-ray intensities of Mg with aninterval of 2 μm, and dividing a standard deviation thereof by anaverage thereof.

The silicon nitride substrate contains grains of silicon nitride andgrain boundary phases mainly containing component added as a sinteringadditive. The grain boundary phase generated with the sintering additiveas a main component serves to retain a bonding force between the siliconnitride grains and to prevent defects between the grains. Particularly,when a coarse defect exists on the surface of the silicon nitridesubstrate, or when a stress is applied to the silicon nitride substrate,the defect may serve as a start point of breakdown. Therefore, it isnecessary to regularly distribute the grain boundary phases to prevent acoarse defect.

Magnesium oxide MgO and yttrium oxide Y₂O₃ used as a sintering additivein the silicon nitride substrate react with Si₃N₄ or SiO₂ contained inSi₃N₄ to form a liquid phase in a sintering process. While the magnesiumoxide MgO helps to generate the liquid phase at a relatively lowtemperate and promote the sintering process, the liquid phase containingMgO is susceptible to volatilization or segregation and is apt toirregularly distribute the grain boundary phases containing Mg generatedfrom the liquid phase particularly on the substrate surface frequentlyexposed to a high temperature in a sintering process. Regularity ofgrain boundary phases containing Mg can be recognized by checking avariation coefficient representing distribution of the amount ofmagnesium Mg on the surface of the silicon nitride substrate.Accordingly, when the variation coefficient representing distribution ofthe amount of magnesium Mg on the surface of the silicon nitridesubstrate is large, a large number of coarse defects are formed on thesurface of the silicon nitride substrate, then bending strengthdecreases, and thus, cracks may be easily generated when a stress isapplied on the silicon nitride substrate during a bonding processbetween: the silicon nitride substrate and the metal circuit plate; andthe silicon nitride substrate and the metal heat sink plate, a processof manufacturing a power semiconductor module, or a heat cycleaccompanied by operation of the power semiconductor module. Accordingly,it is necessary to appropriately adjust the variation coefficientrepresenting distribution of the amount of magnesium Mg on the surfaceof the silicon nitride substrate. On the other hand, distribution of theamount of yttrium Y on the surface of the silicon nitride substrate ishardly influenced by a manufacturing condition.

Therefore, in the silicon nitride substrate according to the presentembodiment, the variation coefficient representing distribution of theamount of magnesium Mg is adjusted to 0.20 or less as described above.Accordingly, it is possible to improve bending strength of the siliconnitride substrate. How to adjust the variation coefficient will bedescribed later.

If warpage of the silicon nitride substrate is large, an adhesionproperty between the silicon nitride substrate and the metal circuitplate or between the silicon nitride substrate and the metal heat sinkplate may be easily degraded in some parts. As a result, the metalcircuit plate and the metal heat sink plate may be easily exfoliatedfrom the silicon nitride substrate. Therefore, in the silicon nitridesubstrate according to the present embodiment, the warpage is limited to2.0 μm/mm or less as described above. How to limit the warpage will bedescribed later.

The silicon nitride substrate according to the present embodimentcontains magnesium Mg of 3.0 to 4.2 wt % as magnesium oxide and yttriumY of 2.0 to 5.0 wt % (preferably, a total of 5.0 to 8.3 wt %) as yttriumoxide. In addition, it is preferable to contain magnesium and yttrium ofcontents to set a ratio of (MgO)/(Y₂O₃) as oxide to 0.62 to 2.2.Magnesium and yttrium function as a sintering additive in a process ofmanufacturing the silicon nitride substrate, and usually exist as agrain boundary phase in the manufactured silicon nitride substrate, andthus, when their contents decrease, coarse defects such as voids may beeasily generated, and bending strength may easily decrease. On the otherhand, when the contents of the magnesium and yttrium increase, a largecontent of grain boundary phases, which has weaker strength comparing tothe silicon nitride grains inside the silicon nitride substrate, areformed. Therefore, breakdown through a grain boundary is easilygenerated and then the bending strength decreases. When the ratiobetween contents of magnesium and yttrium is not within an appropriaterange, they may not sufficiently serve as a sintering additive. As aresult, the sintering of the silicon nitride substrate may not bepromoted, or a weak grain boundary phase may be formed in the siliconnitride substrate, and thus, bending strength decreases. In the presentembodiment, each of the contents of magnesium and yttrium is limited inthe range in order to adjust such properties.

Third Embodiment

Hereinafter, a method of manufacturing a silicon nitride substrateaccording to the first and second embodiments will be described.

FIG. 2 illustrates a flowchart showing a method of manufacturing asilicon nitride substrate according to the present embodiment. Referringto FIG. 2, in a source material adjustment and mixing process (a),magnesium oxide of 3 to 4 wt % and at least one kind of oxide of arare-earth element of 2 to 5 wt % are mixed with silicon nitride sourcematerial powder with a total weight percentage of 5 to 8 wt %. Asolution, an organic binder, a plasticizing material, and the like arealso mixed with them using a ball mill or the like. Here, it ispreferable to use above-described yttrium oxide or the like as at leastone kind of oxide of a rare-earth element.

Then, in a molding process (b), source material slurry obtained by themixture is defoamed and thickened, and then, a sheet molding isperformed to obtain a plate having a predetermined thickness by way of adoctor blade method known in the art. Here, the thickness of thesheet-molded body may be appropriately determined depending on its use.For example, the thickness may be set to 0.1 to 1.0 mm.

Then, in a sintering process (c), the sheet-molded body is inserted intoa furnace and sintered in a nitrogen atmosphere with a pressure of 0.5to 1.0 MPa at a temperature of 1,800 to 2,000 degree C., so as toprovide a silicon nitride substrate.

Then, in a heat treatment process (d), a plurality of silicon nitridesubstrates obtained after the sintering process are stacked, and a heattreatment is performed at a temperature of 1,550 to 1,700 degree C. witha pressure of 0.5 to 6.0 kPa. With such a heat treatment with apressure, it is possible to prevent warpage of the silicon nitridesubstrate and to regularly distribute grain boundary phases includingmagnesium Mg on the substrate surface, thereby improving bendingstrength. If the heat treatment temperature is lower than 1,550 degreeC., the effect of preventing warpage becomes inefficient, and thewarpage of silicon nitride substrate increases. Meanwhile, if the heattreatment temperature is higher than 1,700 degree C., growth of columnargrains contained in the silicon nitride substrate is promoted, then adegree of orientation fa of the silicon nitride substrate increases, andthus, surface roughness increases, further, vaporization or segregationof grain boundary phase components containing magnesium Mg on thesurface of the silicon nitride substrate is promoted, then a coarsedefect is easily generated on the substrate surface, and thus, bendingstrength decreases. Therefore, the heat treatment temperature ispreferable to be in the above-described range. If the pressure appliedduring the heat treatment is lower than 0.5 kPa, the effect ofpreventing warpage is insufficient, and the content of magnesium oxide(MgO) decreases due to vaporization of the grain boundary phasecomponents including magnesium(Mg), and thus, bending strengthdecreases. Meanwhile, if the pressure is higher than 6.0 kPa, the growthof columnar grains contained in the silicon nitride substrate ispromoted, then the degree of orientation fa of the silicon nitridesubstrate increases, and thus, the surface roughness increases. Further,the adhesion property (contact pressure) between substrates may becomeexcessive, then grain boundary phase components, which includemagnesium, are given and received between substrates to promotesegregation, and thus a coarse defect on the surface of the siliconnitride substrate is easily generated to degrade a bending strength.Therefore, the load applied in the heat treatment process is preferablywithin the range described above. Here, the heat treatment is performedwith a plurality of silicon nitride substrate being stacked in order toadjust a vaporization amount of magnesium oxide, yttrium oxide and thelike, which functions as a sintering additive, to control the growth ofcolumnar grains contained in the silicon nitride substrate, and thus tocontrol the degree of orientation fa of the silicon nitride substrate.Furthermore, this is for adjusting the vaporization amount of magnesiumoxide as a sintering additive to control Mg contained on the surface ofthe silicon nitride substrate to be regularly distributed and to adjustthe content of the MgO.

Then, in a blast process (e), abrasive particles are blasted on thesurface of the silicon nitride substrate after the heat treatment togrind the columnar grains existing on the substrate surface so as toreduce surface roughness. The abrasive particles are blasted on bothsurfaces of the silicon nitride substrate.

FIGS. 3 and 4 are illustrative diagrams showing a method of applying apressure during the heat treatment process (d). In FIG. 3, the siliconnitride substrates 10 are interposed between ceramic plate members 14such as boron nitride BN and loaded with a press weight 16. Other thanboron nitride, the plate member 14 may be made of any material, whichdoes not affect composition or the like of the silicon nitride substrateduring the heat treatment process. Generally, boron nitride BN issuitably used among materials readily available. For the press weight16, silicon nitride is preferably used, however, metal having a highmelting point such as tungsten or molybdenum may also be used. Referringto FIG. 4, a hot press 18 is used instead of the press weight 16 toapply a load.

FIG. 5 is an illustrative diagram showing the aforementioned blastprocess. In FIG. 5, abrasive particles 22 are blasted from a nozzle 20onto the surface of the silicon nitride substrate 10 after the heattreatment to grind the surface of the silicon nitride substrate 10.

FIGS. 6A and 6B are electron microscope pictures illustrating thesurface of the silicon nitride substrate before and after the blastprocess. FIG. 6A shows a surface of the silicon nitride substrate beforethe blast process, and FIG. 6B shows a surface of the silicon nitridesubstrate after the blast process. It can be seen that, after the blastprocess in which the abrasive particles 22 are blasted to perform agrinding, columnar grains having a large size on the substrate surfaceare grinded, and then roughness of the substrate surface decreases.

The silicon nitride substrate manufactured as described above exhibitssuperior properties such as high bending strength, a high adhesionproperty with the metal circuit plate and the metal heat sink plate,high fracture toughness. Therefore, the silicon nitride substrate can beused in a circuit board for a high-frequency transistor or a powersemiconductor module, various substrates such as multi-chip modulecircuit board, a thermal conduction plate for a Peltier device, orvarious electronic components such as a heat sink for various heatgenerating elements. If the silicon nitride substrate according to thepresent embodiment is employed in, for example, a substrate for mountinga semiconductor device, it is possible to prevent cracks on thesubstrate in a bonding process of the silicon nitride substrate, themetal circuit board, and the metal heat sink plate; in a process ofmanufacturing a power semiconductor module; or when the substrate issubjected to a repetitive heat cycle accompanied by operation of a powersemiconductor module. Therefore, it is possible to provide a substratehaving thermal shock resistance and heat cycle resistance.

A silicon nitride circuit board is manufactured by bonding a coppercircuit board or an aluminum circuit board, which is a metal circuitplate or a metal heat sink plate, to one or both surfaces of the siliconnitride substrate according to the present embodiment using a directbonding copper (DBC) method or an active metal soldering method or thelike. A typical construction of the silicon nitride circuit boardaccording to the present invention is obtained by bonding the metalcircuit plate to one surface of the silicon nitride substrate accordingto the present embodiment, and bonding the metal heat sink plate to theother surface. In the DBC method, the silicon nitride substrate and thecopper circuit board or an aluminum circuit board are heated within aninert gas or nitride atmosphere at a temperature higher than a processtemperature, and the resultant liquid phase of Cu—O or Al—O eutecticcompounds is used as an adhesive to directly bond a circuit board to oneor both surface of the silicon nitride substrate through an eutecticcompound layer. In the active metal brazing material method, a coppercircuit board or an aluminum circuit board is bonded to one or bothsurface of the silicon nitride substrate by way of a heat and pressprocess within an inert gas or vacuum atmosphere through a brazingmaterial layer obtained by mixing or alloying active metal such astitanium Ti, zirconium Zr, or hafnium Hf with metal such as argentine Agor copper Cu, which makes low melting point alloy with the active metal.After bonding the circuit board, the copper circuit board or thealuminum circuit board on the silicon nitride substrate is etched toform a circuit pattern, and then, an Ni—P plating is performed on thecopper circuit board or the aluminum circuit board having a circuitpattern, and thus a silicon nitride circuit board is manufactured.

In addition, it is possible to manufacture a desired semiconductormodule by mounting semiconductor devices on the silicon nitride circuitboard.

Although the exemplary embodiment of the invention has been describedabove, many changes and modifications will become apparent to thoseskilled in the art in view of the foregoing description which isintended to be illustrative and not limiting of the invention defined inthe appended claims.

EXAMPLES

Now, experimental examples corresponding to the aforementionedembodiments will be described hereinafter, however, the presentinvention is not intended to be limited thereby.

First Example

First Example corresponds to the first embodiment.

A silicon nitride substrate was manufactured based on the manufacturingmethod illustrated in FIG. 2, and its material properties were measured.Each item of manufacturing conditions such as the additive amount ofmagnesium oxide MgO, the additive amount of yttrium oxide Y₂O₃, a heattreatment temperature in the heat treatment process, pressure, whetheror not the silicon nitride substrates are stacked, and the thickness ofthe silicon nitride substrate was established as shown in Table 1(examples 1 to 10). In the Table 1, “Yes” means that the silicon nitridesubstrates are stacked. In the condition where the substrates are notstacked (“No” in the Table 1), a heat treatment process is performed ona single silicon nitride substrate interposed between two boron nitrideBN plate members 14.

Measured material properties of the silicon nitride substrate were adegree of orientation, warpage, surface roughness, bending strength, aWeibull coefficient, fracture toughness, a heat conduction rate, and aheat cycle test result. Specifically, whether each of the warpage, thesurface roughness, the bending strength, and the fracture toughness arewithin a predetermined range (warpage: 2 μm/mm or less, surfaceroughness: 0.44 μm or less, bending strength: 790 MPa or more, andfracture toughness: 6 MPam^(1/2) or more) was determined.

As a comparative example, several extra silicon nitride substrates werealso manufactured under manufacturing conditions different from thefirst example, and their material properties were similarly measured anddetermined. The results are shown in Tables 2-1,2-2 (ComparativeExamples 1 to 13).

In the material properties, the degrees of orientation on the surfaceand inside of the substrate were calculated based on X-ray diffractionbeam intensities using the Equation 1. In addition, as described above,the substrate surface refers to the outermost surface of the siliconnitride substrate obtained after the heat treatment process (d) or thesurface obtained by grinding as deep as 10% or less of the substratethickness from the outermost surface. Here, the outermost surface meansthe surface obtained after the heat treatment process (d), and thesurface obtained by grinding as deep as 10% or less of the substratethickness means the surface obtained after the blast process (e).However, since the degree of orientation is not significantly changedfrom the outermost surface to the surface obtained by grinding as deepas 10% or less of the substrate thickness, the degree of orientation maybe measured on either of the outermost surface or the surface obtainedby grinding as deep as 10% or less of the substrate thickness. In thepresent example, the degree of orientation was measured on the surfaceobtained by grinding as deep as 10% or less of the substrate thicknessfrom the outermost surface (i.e., the surface obtained after the blastprocess (e)). The degree of orientation inside the substrate wasmeasured on the surface obtained by grinding as deep as 20% or more and80% or less of the substrate thickness from the substrate surface. Sincethe degree of orientation inside the substrate can be equally measuredon either surface, the degree of orientation was measured on one surfacein the example. It is to be noted that, although the substrate surfacewas grinded in order to measure the degree of orientation inside thesubstrate in the present example, the substrate surface is not grindedfor that purpose when manufacturing a silicon nitride substrateaccording to the present invention.

The warpage was measured using a 3-dimensional laser meter (trade name:LT-8100, manufactured by KEYENCE). FIGS. 7A and 7B are illustrativediagrams showing a method of measuring warpage. Referring to FIG. 7A, adistance from a suitably selected arbitrary plane to the substratesurface S was measured using a 3-D laser measuring instrument, and thena plane including two points having the shortest distance was selectedas a reference plane. Subsequently, the height D of the highest pointhaving largest height (distance) from the reference plane was defined asa warpage amplitude. In addition, as shown in FIG. 7B, the measurementof the distance from the arbitrary plane to the substrate surface S wasperformed on a diagonal line of the silicon nitride substrate. A warpageamount was obtained by dividing the warpage amplitude by a scanningdistance (i.e., a diagonal length shown in FIG. 7B).

The surface roughness was measured for several arbitrary locations onthe substrate surface using a surface roughness meter in accordance witha standard JIS-B0601, then an arithmetic mean value of roughness Ra wasobtained.

The bending strength was measured by a three-point bending test inaccordance with a standard JIS-B1601. Specifically, a specimen of thesilicon nitride substrate having a width of 4 mm was prepared, and seton a three-point bending jig having a support roll distance of 7 mm.Then, a load was applied at a crosshead speed of 0.5 mm/min, and a loadapplied to the specimen at the time point of breakdown was used tocalculate the bending strength.

The Weibull coefficient was obtained by drawing a Weibull plot byplotting lnln(1−F)⁻¹ with respect to lnσ based on the bending strengthtest result in accordance with a standard JIS-R1625 and calculating aninclination of the plot. Here, a denotes bending strength, and F denotesan accumulative breakdown probability.

The fracture toughness was measured using an IF method in which aVickers indenter was pressed into the side face of the silicon nitridesubstrate with a predetermined load (e.g., in this example, 2 kgf(19.6N)) in accordance with a standard JIS-R1607. In this case, theVickers indenter was pressed into the side face such that a diagonalline of Vickers indentation is perpendicular to the thickness directionof the silicon nitride substrate. The surface used to measure fracturetoughness was obtained by cutting out the silicon nitride substrate andthen performing a mirror-polishing on the obtained surface.

The heat conduction rate was measured in accordance with a standardJIS-R1611 by cutting out a measurement specimen having a width of 5 mmfrom the silicon nitride substrate.

The heat cycle test was performed by repeating a heating/cooling cycleincluding cooling at −55 degree C. for 20 minutes, annealing at a roomtemperature for 10 minutes, heating at 150 degree C. for 20 minutes3,000 times, and then, determining pass or fail based on whether or notbreakdown of the silicon nitride substrate or exfoliation of the metalcircuit plate or the metal heat sink plate is generated. Here, thefurnace (trade name: TSA-101S-W, manufactured by ESPEC Co.) was used toperform the heating/cooling cycle.

[Table 1] [Tables 2-1,2-2]

As shown in Table 1, a silicon nitride substrate having a thickness of0.1 to 1.0 mm was manufactured under the conditions that the additiveamount of magnesium oxide is 3 to 4 wt %, the additive amount of yttriumoxide Y₂O₃ is 2 to 5 wt %, a heat treatment temperature in the heattreatment process is 1,550 to 1,700 degree C., a pressure is 0.5 to 6.0kPa and substrates are stacked. The manufactured silicon nitridesubstrate exhibits satisfactory material properties including the degreeof orientation of the substrate surface being within the range: 0.33 orless, the degree of orientation inside the substrate being within therange: 0.16 to 0.33, the surface roughness being within the range: 44 μmor less, the warpage being within the range: 2 μl/mm or less, thebending strength being within the range: 790 MPa or more, the fracturetoughness being within the range: 6 MPam^(1/2) or more. In addition, itis recognized that the Weibull coefficient was also satisfactory beingwithin the range: 15 or more, and a variation of the bending strengthwas also small. As a result, even in a heat cycle test, all specimenswere determined to pass without breakdown of the silicon nitridesubstrate or exfoliation of the metal circuit plate or the metal heatsink plate.

In a heat treatment process, since each outer surface of the uppermostand lowermost substrates out of a plurality of stacked silicon nitridesubstrates makes contact with a corresponding plate member 14,vaporization of the sintering additive such as MgO or Y₂O₃ is promotedon the contact surface with the plate member 14. However, since eachinner surface of the uppermost and lowermost substrates also makescontact with other silicon nitride substrates, vaporization is preventedon these contact surfaces, and thus material properties are notsignificantly degraded. The reason why vaporization of the sinteringadditive is promoted on the contact surface with the plate member 14will be described later.

On the other hand, as shown in Table 2-1, a silicon nitride substratehaving a thickness of 0.32 mm was manufactured as a comparative example1 under the conditions that the additive amount of MgO is 3 wt %, theadditive amount of Y₂O₃ is 2 wt %, and a heat treatment is notperformed. The manufactured silicon nitride substrate exhibited highwarpage as much as 2.9 μm/mm. This is because the warpage of the siliconnitride substrate could not be restrained since a heat treatment was notperformed. As a result, exfoliation of the metal circuit plate or themetal heat sink plate was generated in the heat cycle test.

As a comparative example 2, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 3 wt %, the additive amount of Y₂O₃ is 2 wt %,the heat treatment temperature is 1,450 degree C., the pressure is 2.2kPa, and the substrates are stacked. While the heat treatment wasperformed unlike the comparative example 1, the temperature was low.Therefore, the warpage increased to 2.5 μm/mm. As a result, exfoliationof the metal circuit plate or the metal heat sink plate was generated inthe heat cycle test.

As a comparative example 3, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 3 wt %, the additive amount of Y₂O₃ is 2 wt %,the heat treatment temperature is 1,800 degree C., the pressure is 2.4kPa, and the substrates are stacked. In the manufactured silicon nitridesubstrate, the degree of orientation increased to 0.34, and the surfaceroughness on the substrate surface increased to 0.45 μm. This is becausegrowth of columnar grains of silicon nitride is promoted by a hightemperature heat treatment, and the content of columnar grains having alarge longitudinal length increases. As a result, exfoliation of themetal circuit plate or the metal heat sink plate was generated in theheat cycle test. A silicon nitride substrate of a comparative example 4was manufactured by changing the additive amount of Y₂O₃ to 3 wt % andthe pressure to 2.6 kPa unlike the comparative example 3, and a siliconnitride substrate of a comparative example 5 was manufactured bychanging the additive amount of MgO to 4 wt % and the pressure to 1.9kPa unlike the comparative example 3. In both comparative examples 4 and5, the heat treatment temperature increased to 1,800 degree C., thedegree of orientation of the substrate surface increased to 0.38 in thecomparative example 4 and to 0.35 in the comparative example 5, and thesurface roughness increased to 0.46 p in the comparative example 4 andat 0.47 μm in the comparative example 5. As a result, exfoliation of themetal circuit plate or the metal heat sink plate was generated in theheat cycle test.

As a comparative example 6, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 3 wt %, the additive amount of Y₂O₃ is 2 wt %,the heat treatment temperature is 1,600 degree C., the pressure is 6.5kPa, and the substrates was stacked. In the manufactured silicon nitridesubstrate, the degree of orientation increased to 0.35, and the surfaceroughness increased to 0.45 μm on the substrate surface. This isbecause, since the pressure increased to 6.5 kPa during the heattreatment, vaporization of the sintering additive MgO or Y₂O₃ isrestrained on the contact surfaces between the silicon nitridesubstrates, and growth of columnar grains of silicon nitride ispromoted, and thus the content of columnar grains having a longlongitudinal length increases. As a result, exfoliation of the metalcircuit plate or the metal heat sink plate was generated in the heatcycle test.

As a comparative example 7, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 3 wt %, the additive amount of Y₂O₃ is 2 wt %,the heat treatment temperature is 1,600 degree C., no pressure isapplied, and the substrates are stacked. Since there was no pressureduring the heat treatment process, the effect of preventing warpage wasinsufficient, and the warpage increased to 3.0 μm/mm. As a result,exfoliation of the metal circuit plate or the metal heat sink plate wasgenerated in the heat cycle test.

As a comparative example 8, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 3 wt %, the additive amount of Y₂O₃ is 3 wt %,the heat treatment temperature is 1,600 degree C., the pressure is 2.1kPa, and the substrates is not stacked. In the manufactured siliconnitride substrate, the bending strength and the fracture toughnessdecrease to 780 MPa and 5.9 MPam^(1/2), respectively. This is because,since the silicon nitride substrates are not stacked in the heattreatment process, vaporization of the sintering additive MgO or Y₂O₃ ispromoted, and growth of columnar grains of silicon nitride is restrainedparticularly inside the silicon nitride substrate, and thus the contentof columnar grains having a short longitudinal length increases. As aresult, in the comparative example, a degree of orientation inside thesilicon nitride substrate decreases to 0.15. Accordingly, breakdown(e.g., cracks) of the silicon nitride substrate is generated in the heatcycle test. In the comparative example, a single silicon nitridesubstrate is interposed between two boron nitride plate members 14.Since the boron nitride has a density of about 80% and has a number ofvoids, it is guessed that the sintering additive vaporized from thesilicon nitride substrate during the heat treatment process is absorbedinto the boron nitride members or vaporized to an atmosphere through theboron nitride members.

As a comparative example 9, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 3 wt %, the additive amount of Y₂O₃ is 3 wt %,the heat treatment temperature is 1,600 degree C., no pressure isapplied, and the substrates are not stacked. Since there was no pressurein the heat treatment process, the warpage increases to 3.2 μm/mm. Sincethe silicon nitride substrates are not stacked, the degree oforientation inside the silicon nitride substrate decreased to 0.15, andthe bending strength decreases to 788 MPa. As a result, exfoliation ofthe metal circuit plate or the metal heat sink plate was generated inthe heat cycle test.

As a comparative example 10, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 3 wt %, the additive amount of Y₂O₃ is 1 wt %,the heat treatment temperature is 1,600 degree C., a pressure is 3.0kPa, and the substrates are stacked. In the manufactured silicon nitridesubstrate, the degree of orientation inside the manufactured siliconnitride substrate decreased to 0.14, and also both of the bendingstrength and the fracture toughness decreased to 734 MPa and 5.1MPam^(1/2), respectively. This is because, since the additive amount ofY₂O₃ is small (1 wt %), growth of columnar grains of silicon nitride isrestrained particularly inside the silicon nitride substrate, and thecontent of columnar grains having a short longitudinal length increases.Accordingly, in the comparative example, the degree of orientationinside the silicon nitride substrate decreases to 0.14. As a result,breakdown (cracks) of the silicon nitride substrate is generated in theheat cycle test.

As a comparative example 12 (Table 2-2), a silicon nitride substratehaving a thickness of 0.32 mm was manufactured under the conditions thatthe additive amount of MgO is 2 wt %, the additive amount of Y₂O₃ is 2wt %, the heat treatment temperature is 1,600 degree C., a pressure is3.5 kPa, and the substrates are stacked. Similar to the comparativeexample 10, the degree of orientation inside the silicon nitridesubstrate decreased to 0.14, and the bending strength and the fracturetoughness decreased to 767 MPa and 5.8 MPam¹², respectively. This isbecause, since the additive amount of MgO functioning as a sinteringadditive is small (2 wt %), growth of columnar grains of silicon nitrideis restrained particularly inside the silicon nitride substrate, and thecontent of columnar grains having a short longitudinal length increases.Accordingly, in the comparative example, the degree of orientationinside the silicon nitride substrate decreases to 0.14. As a result,breakdown (cracks) of the silicon nitride substrate is generated in theheat cycle test.

When the degree of orientation inside the substrate is smaller than 0.16as the comparative examples 8, 9, 10, and 12, the ratio of columnargrains having a long length decreases, and the bending strength and thefracture toughness decrease, and thus the silicon nitride substrate maybecome susceptible to cracks during a bonding process of the siliconnitride substrate to the metal circuit plate and the metal heat sinkplate, or a heat cycle accompanied by operation of a power semiconductormodule.

As a comparative example 11 (Table 2-2), a silicon nitride substratehaving a thickness of 0.32 mm was manufactured under the conditions thatthe additive amount of MgO is 3 wt %, the additive amount of Y₂O₃ is 6wt %, the heat treatment temperature is 1,600 degree C., a pressure is2.1 kPa, and the substrates are stacked. In the manufactured siliconnitride substrate, the degree of orientation increases both on thesurface of the substrate and inside the substrate to 0.39 and 0.34,respectively, and the surface roughness on the substrate surfaceincreases to 0.46 μm. This is because, since the additive amount of Y₂O₃functioning as a sintering additive is large at 6 wt %, growth ofcolumnar grains of silicon nitride is promoted, and the content ofcolumnar grains having a long longitudinal length increases. As aresult, exfoliation of the metal circuit plate or the metal heat sinkplate was generated in the heat cycle test.

As a comparative example 13 (Table 2-2), a silicon nitride substratehaving a thickness of 0.32 mm was manufactured under the conditions thatthe additive amount of MgO is 5 wt %, the additive amount of Y₂O₃ is 3wt %, the heat treatment temperature is 1,600 degree C., a pressure is2.3 kPa, and the substrates are stacked. Also in the manufacturedsilicon nitride substrate, the degree of orientation increased both onthe surface of the substrate and inside the substrate to 0.40 and 0.35,respectively, and the surface roughness of the substrate surfaceincreased to 0.45 μm. This is because, since the additive amount of MgOfunctioning as a sintering additive increases to 5 wt %, growth ofcolumnar grains of silicon nitride is promoted, and the content ofcolumnar grains having a long longitudinal length increases. As aresult, exfoliation of the metal circuit plate or the metal heat sinkplate was generated in the heat cycle test.

When the degree of orientation inside the substrate is larger than 0.33as in the comparative examples 11 and 13, the degree of orientation faon the surface of the silicon nitride substrate also increases. When thedegree of orientation is large both on the surface of the substrate andinside the substrate, grain growth is processed on the entire substrate,and thus, a coarse defect can be easily generated. Since this defect mayserve as a start point to generate the breakdown or to make thebreakdown easily progress, the bending strength of the substratedecreases.

As described above, when the silicon nitride substrate is manufacturedunder the conditions specified in Table 1, the degree of orientation andother material properties are within the ranges specified in Table 1,and thus, breakdown of the silicon nitride substrate, or exfoliation ofthe metal circuit plate or the metal heat sink plate is not generated.However, when any item of the manufacturing conditions is not within therange, breakdown of the silicon nitride substrate, or exfoliation of themetal circuit plate or the metal heat sink plate may be generated.

Second Example

Second Example corresponds to the second embodiment.

A silicon nitride substrate was manufactured based on the manufacturingmethod illustrated in FIG. 2, and its material properties were measured.Each item of the manufacturing conditions such as the additive amount ofmagnesium oxide MgO, the additive amount of yttrium oxide Y₂O₃, a totaladditive amount of MgO and Y₂O₃, a heat treatment temperature in theheat treatment process, pressure, whether or not the silicon nitridesubstrates are stacked, and the thickness of the silicon nitridesubstrate was established as shown in Table 3 (examples 11 to 20). Inthe Table 3, “Yes” means that the silicon nitride substrates arestacked. In the condition where the substrates are not stacked (“No” inthe Table 3), a heat treatment process is performed on a single siliconnitride substrate interposed between two boron nitride BN plate members14.

Measured material properties of the silicon nitride substrate were avariation coefficient of magnesium on the surface of the silicon nitridesubstrate, a content of magnesium oxide MgO, a content of yttrium oxideY₂O₃, a ratio of contents MgO/Y₂O₃, a total content of MgO and Y₂O₃,warpage, bending strength, a Weibull coefficient, fracture toughness, aheat conduction rate and heat shock test result. Specifically, whethereach of the warpage, the bending strength, and the fracture toughness iswithin a predetermined range (warpage: 2 μm/mm or less, bendingstrength: 820 MPa or more, and fracture toughness: 6 MPam^(1/2) or more)was determined.

As a comparative example, several extra silicon nitride substrates werealso manufactured under manufacturing conditions different from thesecond example, and their material properties were similarly measuredand determined. The results are shown in Table 4 (Comparative Examples14 to 26).

In the material properties, the variation coefficient of magnesium wasobtained by performing an EMPA analysis on the substrate surface asdescribed above. In addition, as described above, the substrate surfacerefers to the outermost surface of the silicon nitride substrate or thesurface obtained by grinding as deep as 10% or less of the substratethickness (15 (μm at maximum) from the outermost surface. Here, theoutermost most surface means the surface obtained after the heattreatment, and the surface obtained by grinding as deep as 10% or lessof the substrate thickness (15 (μm at maximum) means the surfaceobtained after the blast process. Since the variation coefficient is notsignificantly changed from the outermost surface to the surface obtainedby grinding as deep as 10% or less of the substrate thickness, thevariation coefficient may be measured on either of the outermost surfaceor the surface obtained by grinding as deep as 10% or less of thesubstrate thickness. In the present example, the variation coefficientof magnesium was measured on the surface obtained by grinding as deep as10% or less of the substrate thickness from the outermost surface (i.e.,the surface obtained after the blast process).

The content of magnesium oxide MgO and the content of yttrium oxide Y₂O₃were obtained by changing the phase of the nitride substrate intosolution using microwave decomposition process and acid dissolutionprocess, measuring the contents of magnesium and yttrium using ICPemission spectrometry, and then converting the contents in magnesiumoxide equivalent and yttrium oxide equivalent. The ratio of contentsMgO/Y₂O₃, and the total content of MgO and Y₂O₃ were calculated based onthe obtained content of MgO and the content of Y₂O₃.

The warpage was measured using a 3-dimensional laser meter (trade name:LT-8100, manufactured by KEYENCE) similar to the example 1. A method ofmeasuring the warpage is similar to that described in association withFIGS. 7A and 7B. A warpage amount was obtained by dividing the warpageamplitude by a scanning distance (i.e., a diagonal length shown in FIG.7B).

The bending strength was measured by a three-point bending test inaccordance with a standard JIS-R1601 in a similar way to that of theexample 1.

The Weibull coefficient was obtained based on the bending strength testresult in accordance with a standard JIS-R1625 in a similar way to thatof the example 1.

The fracture toughness was measured using an IF method in accordancewith a standard JIS-R1607 in a similar way to that of the example 1.

The heat conduction rate was measured in accordance with a standardJIS-R1611 by cutting out a measurement specimen having a width of 5 mmfrom the silicon nitride substrate in a similar way to that of theexample 1.

In the heat shock test, the silicon nitride, first surface on which, acopper circuit plate is formed and second surface on which, a copperheat sink plate is formed, is kept at a temperature of 350 degree C. for10 minutes and quickly cooled to a room temperature, and then whetherthere is a crack on the silicon nitride substrate or not is examined.This test was repetitively performed 10 times to determine whether thesilicon nitride substrate is passed or failed based on whether or not acrack is generated. When the warpage is larger than 2.0 μm/mm, thesubstrate cannot be used for a silicon nitride circuit board. Therefore,there is no need to perform the heat shock test.

[Table 3]

[Table 4]

As shown in Table 3, a silicon nitride substrate having a thickness of0.1 to 1.0 mm was manufactured under the conditions that the additiveamount of magnesium oxide MgO is 3 to 4 wt %, the additive amount ofyttrium oxide Y₂O₃ is 2 to wt %, a heat treatment temperature is 1,550to 1,700 degree C., a pressure is 0.5 to 6.0 kPa, and substrates arestacked. The manufactured silicon nitride substrate exhibitssatisfactory material properties including the variation coefficient ofmagnesium being within the range: 0.20 or less, the content of MgO beingwithin the range: 3.0 to 4.2 wt %, the content of Y₂O₃ being within therange: 2.0 to 5.0 wt %, the ratio of contents MgO/Y₂O₃ being within therange: 0.62 to 2.2, a total content of MgO and Y₂O₃ being within therange: 5.0 to 8.3 wt %, the warpage being within the range: 2 μm/mm orless, the bending strength being within the range: 820 MPa or more, andthe fracture toughness being within the range: 6 MPam^(1/2) or more. Inaddition, it is recognized that the Weibull coefficient was alsosatisfactory being within the range: 15 or more, and a variation of thebending strength was also small. As a result, even in a heat shock test,all specimens were determined to pass without breakdown of the siliconnitride substrate.

In a heat treatment process, since each outer surface of the uppermostand lowermost substrates out of a plurality of stacked silicon nitridesubstrates makes contact with a corresponding plate member 14,vaporization of the sintering additive such as MgO or Y₂O₃ is promotedon the contact surface with the plate member 14. However, since eachinner surface of the uppermost and lowermost substrates also makescontact with other silicon nitride substrates, vaporization is preventedon these content surfaces, and thus material properties are notsignificantly degraded. The reason why vaporization of the sinteringadditive is promoted on the contact surface with the plate member 14will be described later.

On the other hand, as shown in Table 4, a silicon nitride substratehaving a thickness of 0.32 mm was manufactured as a comparative example14 under the conditions that the additive amount of MgO is 3 wt %, theadditive amount of Y₂O₃ is 2 wt %, and a heat treatment is notperformed. The manufactured silicon nitride substrate exhibited highwarpage as much as 2.9 μm/mm. This is because the warpage of the siliconnitride substrate could not be restrained since a heat treatment was notperformed. Also, the variation coefficient of magnesium increased to0.45, and the bending strength decreased to 812 MPa. This is because theheat treatment was not performed, and thus magnesium is not regularlydistributed on the substrate surface.

As a comparative example 15, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 3 wt %, the additive amount of Y₂O₃ is 2 wt %,the heat treatment temperature is 1,450 degree C., the pressure is 2.2kPa, and the substrates are stacked. While the heat treatment wasperformed unlike the comparative example 14, the heat treatmenttemperature was low. Therefore, the warpage increased to 2.5 μm/mm.

As a comparative example 16, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 3 wt %, the additive amount of Y₂O₃ is 2 wt %,the heat treatment temperature is 1,800 degree C., the pressure is 2.4kPa, and the substrates are stacked. In the manufactured silicon nitridesubstrate, the variation coefficient of magnesium increased to 0.32, andthe bending strength decreased to 795 MPa. This is because vaporizationor segregation of grain boundary phase components on the surface of thesilicon nitride substrate is promoted by a heat treatment at a hightemperature. As a result, cracks were generated on the silicon nitridesubstrate in the heat shock test. A silicon nitride substrate of acomparative example 17 was manufactured by changing the additive amountof Y₂O₃ to 3 wt % and the pressure to 2.6 kPa unlike the comparativeexample 16, and a silicon nitride substrate of a comparative example 18was manufactured by changing the additive amount of MgO to 4 wt % andthe pressure to 1.9 kPa unlike the comparative example 16. In bothcomparative examples 17 and 18, the heat treatment temperature increasedto 1,800 degree C., the variation coefficient of magnesium on thesubstrate surface increased to 0.42 in the comparative example 17 and to0.27 in the comparative example 18, and the bending strength decreasedto 812 MPa in the comparative example 17 and to 808 MPa in thecomparative example 18. As a result, a crack was generated on thesilicon nitride substrate in the heat shock test.

As a comparative example 19, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 3 wt %, the additive amount of Y₂O₃ is 2 wt %,the heat treatment temperature is 1,600 degree C., the pressure is 6.5kPa, and the substrates are stacked. In the manufactured silicon nitridesubstrate, the variation coefficient of magnesium on the substratesurface increased to 0.46, and the bending strength was low at 798 MPa.This is because, since the pressure during the heat treatment increasedto 6.5 kPa, segregation of grain boundary phase components on thesilicon nitride substrate was promoted. As a result, a crack wasgenerated on the silicon nitride substrate in the heat shock test.

As a comparative example 20, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 3 wt %, the additive amount of Y₂O₃ is 2 wt %,the heat treatment temperature is 1,600 degree C., no pressure isapplied, and the substrates are stacked. Since there was no pressureduring the heat treatment process, the effect of preventing warpage wasinsufficient, and the warpage increased to 3.0 μm/mm. In addition, thecontent of MgO decreased to 2.9 wt %, and the bending strength decreasedto 802 MPa. This is because vaporization of the grain boundary phasecomponents is promoted on the surface of the silicon nitride substratedue to the heat treatment without a pressure.

As a comparative example 21, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 3 wt %, the additive amount of Y₂O₃ is 3 wt %,the heat treatment temperature is 1,600 degree C., the pressure is 2.1kPa, and the substrates are not stacked. In the manufactured siliconnitride substrate, the variation coefficient of magnesium on thesubstrate surface increased to 0.23, the content of MgO decreased to 2.9wt %, the bending strength decreased to 780 MPa, and the fracturetoughness decreased to 5.9 MPam^(1/2). This is because, since thesilicon nitride substrates are not stacked in the heat treatmentprocess, vaporization or segregation of grain boundary phase componentsis promoted. As a result, a crack was generated on the silicon nitridesubstrate in the heat shock test. In the comparative example, a singlesilicon nitride substrate is interposed between two boron nitride BNplate members 14. Since the boron nitride has a density of about 80% andhas a number of voids, it is guessed that the sintering additivevaporized from the silicon nitride substrate during the heat treatmentprocess is absorbed into the boron nitride members or vaporized to anatmosphere through the boron nitride members.

As a comparative example 22, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 3 wt %, the additive amount of Y₂O₃ is 3 wt %,the heat treatment temperature is 1,600 degree C., a pressure is notapplied, and the substrates are not stacked. Since there was no pressurein the heat treatment process, the warpage increased to 3.2 μm/mm. Sincethe silicon nitride substrates are not stacked in the heat treatment,vaporization or segregation of grain boundary phase components ispromoted, the variation coefficient of magnesium on the substratesurface increased to 0.23, the content of MgO decreased to 2.9 wt %, andthe bending strength decreased to 788 MPa.

As a comparative example 23, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 3 wt %, the additive amount of Y₂O₃ is 1 wt %,the heat treatment temperature is 1,600 degree C., a pressure is 3.0kPa, and the substrates are stacked. In the manufactured silicon nitridesubstrate, the bending strength and the fracture toughness decreased to734 MPa and 5.1 MPam^(1/2), respectively. This is because, since theadditive amount of Y₂O₃ functioning as a sintering additive is small (1wt %), the content of Y₂O₃ decreases to 1.0 wt %, a total content of MgOand Y₂O₃ decreases to 4.1 wt %, and the a ratio of contents MgO/Y₂O₃increases to 3.2, grain boundary phase components, which have weakstrength and have a number of defect, were formed. As a result, a crackis generated on the silicon nitride substrate in the heat shock test.

As a comparative example 24, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 3 wt %, the additive amount of Y₂O₃ is 6 wt %,the heat treatment temperature is 1,600 degree C., a pressure is 2.1kPa, and the substrates are stacked. In the manufactured silicon nitridesubstrate, since the additive amount of Y₂O₃ functioning as a sinteringadditive increased to 6 wt %, the content of Y₂O₃ also increased to 6.0wt %, and a total content of MgO and Y₂O₃ increased to 9.1 wt %. Inaddition, since a ratio of contents MgO/Y₂O₃ decreased to 0.52, a numberof grain boundary phase components having a weak strength were formed.As a result, a crack was generated on the silicon nitride substrateduring the heat shock test.

As a comparative example 25, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 2 wt %, the additive amount of Y₂O₃ is 2 wt %,the heat treatment temperature is 1,600 degree C., a pressure is 3.0kPa, and the substrates are stacked. In the manufactured silicon nitridesubstrate, the bending strength and the fracture toughness decreased to767 MPa and 5.8 MPam^(1/2), respectively. This is because, since theadditive amount of MgO functioning as a sintering additive decreased to2 wt %, the content of MgO decreased to 2.1 wt %, and a total content ofMgO and Y₂O₃ decreased to 4.1 wt %, and thus grain boundary phasecomponents having a number of defects were formed. As a result, a crackwas generated on the silicon nitride substrate during the heat shocktest.

As a comparative example 26, a silicon nitride substrate having athickness of 0.32 mm was manufactured under the conditions that theadditive amount of MgO is 5 wt %, the additive amount of Y₂O₃ is 3 wt %,the heat treatment temperature is 1,600 degree C., a pressure is 2.3kPa, and the substrates are stacked. In the manufactured silicon nitridesubstrate, since the additive amount of MgO functioning as a sinteringadditive increased to 5 wt %, the content of MgO also increased to 5.2wt %, and a number of grain boundary phase components having weakstrength were generated. As a result, a crack was generated on thesilicon nitride substrate during the heat shock test.

As described above, when the silicon nitride substrate is manufacturedunder the conditions specified in Table 3, the variation coefficient ofmagnesium and other material properties are within the ranges specifiedin Table 3, and thus no crack or breakdown is generated on the siliconnitride substrate. However, it was found out that, when any item of themanufacturing conditions is not within the range, the warpage of thesilicon nitride substrate increases, or breakdown of the silicon nitridesubstrate is generated.

1. A silicon nitride substrate containing silicon nitride, wherein adegree of orientation representing an orientation ratio on a planeperpendicular to a thickness direction, determined by a ratio of X-raydiffraction beam intensity on a predetermined lattice surface of a grainof the silicon nitride is 0.33 or less on a surface; and the degree oforientation is 0.16 to 0.33 on a surface obtained by grinding as deep as20% or more of a thickness of the substrate from a substrate surface;and warpage is 2.0 μm/mm or less.
 2. The silicon nitride substrateaccording to claim 1, wherein the silicon nitride contains magnesium of3 to 4 wt % as magnesium oxide MgO and yttrium of 2 to 5 wt % as yttriumoxide Y₂O₃.
 3. A silicon nitride circuit board including a metal circuitplate bonded to one surface of the silicon nitride substrate accordingto claim 1, and a metal heat sink plate bonded to the other surfacethereof.
 4. A semiconductor module including the silicon nitride circuitboard according to claim 3 and a semiconductor device mounted on thesilicon nitride circuit board.
 5. A silicon nitride substrate containingP-type silicon nitride, yttrium Y, and magnesium Mg, wherein a variationcoefficient representing distribution of magnesium on a surface of thesilicon nitride substrate is 0.20 or less, and warpage is 2.0 μm/mm orless.
 6. The silicon nitride substrate according to claim 5, wherein acontent of magnesium Mg is 3.0 to 4.2 wt % as magnesium oxide MgO, and acontent of yttrium Y is 2.0 to 5.0 wt % as yttrium oxide Y₂O₃.
 7. Asilicon nitride circuit board including a metal circuit plate bonded toone surface of the silicon nitride substrate according to claim 5, and ametal heat sink plate bonded to the other surface thereof.
 8. Asemiconductor module including the silicon nitride circuit boardaccording to claim 7 and a semiconductor device mounted on the siliconnitride circuit board.
 9. A method of manufacturing a silicon nitridesubstrate, comprising: mixing magnesium oxide of 3 to 4 wt % and atleast one kind of rare-earth element oxide of 2 to 5 wt % with siliconnitride source material powder to form a sheet-molded body; sinteringthe sheet-molded body; and performing a heat treatment at a temperatureof 1,550 to 1,700 degree C. with a pressure of 0.5 to 6.0 kPa with aplurality of substrates being stacked.
 10. The method of manufacturing asilicon nitride substrate according to claim 9, wherein abrasiveparticles are blasted on a surface of the silicon nitride substrateafter the heat treatment to grind columnar grains existing on thesurface of the silicon nitride substrate.